Towards Patient-Specific and Reliable SAR Management for Parallel-Transmit Technology on Ultra-High Field MRI

Lead Research Organisation: Lancaster University
Department Name: Engineering

Abstract

Neural circuitry inside a human brain is probably the most complicated large scale circuit that any electronic engineer could think of: it contains around100 billion neurons, instantly linked through trillions of pathways to generate thought, memory, action or emotion. When faults rise within the neural circuitry, different brain disorders may arise. There are 10 million people in the UK living with a neurological condition, and the associated economic burden is estimated to be 116 billion pounds per year. With the recent significant investments in Brain Mapping projects in both Europe and the US, Neuroscientists need to develop new imaging technologies with superior signal-to-noise ratio (SNR), such as Ultrahigh field Magnetic Resonance Imaging, to show how complex neural circuits interact and eventually yield methods of preventing and treating neurological disorders.

However various image artifacts presented at 7T MR images raise uncertainty of MRI measurements, because of increased inhomogeneity of the magnetic excitation field. The latest technology to solve this problem is to utilise a novel RF technology: Parallel Transmission (pTX), through multiple RF excitation coils with unique spatial profiles to achieve the homogeneous magnetization. To adopt this engineering solution for medical imaging application, there is a preeminent need to address one technical challenge: to develop a reliable RF safety management, measured by Specific Absorption Rate (SAR), to ensure this technique will meet the RF exposure legal constraint. Since the advent of pTX for 7T MRI, the difficulty in accurately characterising the local SAR, especially for the pTX pulse optimised for a specific subject, greatly hampers its clinical application. The current practice for SAR estimation in use worldwide by adopting electromagnetic (EM) simulation requires an expensive safety margin (at least 40% higher) to accommodate the uncertainties. The other SAR calculation approach recently pioneered by our project partner (Philips Healthcare), B1-map based ultrafast SAR calculation, still suffers from poor accuracy due to several unrealistic default assumptions, and has not been validated against quantitative experimental measurements at Ultrahigh field MRI yet.

Within this project, we will develop a trustworthy and ultrafast SAR calculation approach to address this Engineering challenge by combining the electromagnetic numerical solution with B1 mapping for the first time, to enable a paradigm shift from using numerical simulation with generic models to proactively managing SAR for specific subjects under examination. In particular we will tackle the poor accuracy problem associated with the latest MRI B1-map based ultrafast SAR calculation by using complementary EM knowledge to supplement the missing information. It will be the first attempt to adopt such a hybrid approach, which could draw the essence of each technique, to tackle the MRI SAR issue in this global research forefront area.

The project is seen as timely because it will address two challenges raised in the 2012 EPSRC/MRC Medical Imaging Technology Working Group Report: "Safer, lower cost, and higher throughput systems and Improving the value of current medical imaging technologies". The research project will also benefit from close collaboration with leading national and international partners in MRI RF technology from both academia (Nottingham University and Queensland University) and industry (Both Philips Healthcare and Pulseteq Ltd) to ensure its immediate impact on Healthcare Industry in the UK from the outset. To accelerate the impact of our research, the SAR management software developed here will provide free licences to all academic and clinical MR researchers. The applicant will also engage actively with the Health Protection Agency's Advisory Group, British Institute of Radiology and Safety Committee of the ISMRM for their reviews on our research output.

Planned Impact

Impact on Healthcare Sector:
There are 10 million people in the UK living with a neurological condition and the associated economic burden is about 116 billion pounds per year. Ultrahigh Field MRI will provide unique clinical information that is elusive at lower field strength, and become a valuable tool for early prevention, detection and treating these disorders. However clinical application of 7T pTX technology is significantly constrained by the RF challenge related with local SAR. For the RF safety reason, the RF pulse designed for 7T pTX technology usually sacrifices its effectiveness, and therefore impedes the high-speed acquisition. The SAR calculation approach proposed here will revolutionise the widely-used SAR calculation methods and address this technical bottleneck facing 7T. This ultrafast approach will increase patient throughput and save scan preparation time -all key agenda items in healthcare.
This proposal is made at a critical time associated with significant investment in Ultrahigh Field MRI facilities in the UK (two multi-million-pound 7T MRI National Facility investment in the past 6 years) and at a time of worldwide investment in Brain Mapping Project. Our research meets this need and has significant potential international impact. We will closely collaborate with clinical end-users working on the 7T national facility to ensure the impact is direct and immediate. Different communication channels, (including a dedicated website, a workshop, software downloads and publications) will be utilised to promote our research outputs to a broad audience. Please refer to the "Pathway to Impact" document for details.

Impact on Commercial Private Sector:
Ultrahigh Field MRI is a topical R&D theme for the medical device industry and Big MR vendors (Philips and Siemens) all rushed to commercialize 7T whole-body systems. There is a strong market demand for pTX RF hardware dedicated to Ultrahigh field MRI, especially for non-neuroimaging applications (e.g. dynamic MR body imaging). Historically, the UK has played an instrumental role in research-led MRI hardware commercialisation. It is very important that UK's world-leading position in MRI commercialization could be maintained, and one of the key strategies is to snap this excellent business opportunity in the highly profitable medical equipment industry for RF hardware developers in the UK. The robust SAR management tool developed here will be able to help commercial MRI RF developers to characterise hardware's performance, validate the safety margin associated with specific hardware, and assist its wide adoption by more medical imaging institutions. To accelerate the impact of this project on UK business and economy, we will closely collaborate with our industrial partners, Philips Helthcare and Pulseteq Ltd., to advance their R&D activity on SAR-constrained pTX technology for both 3T and 7T body imaging. Several measures as described in the impact plan (including periodical review meetings, flexible secondment to companies) will be taken to ensure the impact could be achieved.

Impact on National and International Safety Agencies for Non-ionizing Radiation:
Legislation of Ultrahigh Field MRI safety is the major obstacle for wide clinical adoption of pTX technology. Currently, safety agencies for Non-ionizing Radiation focus on addressing the safety issues associated with the pTX at 7T. The SAR measurement strategy proposed here with quantitative measurements/verification will serve as a scientific guide for them to set reasonable safety measures/regulation for this technology. We will actively engage with key organisations for Non-ionising Radiation Safety through our collaborators from Queensland, Philips and Nottingham. We will invite key figures from these organisations, e.g. AGNIR, BIR and ICNIRP, our project partners and clinical medical imaging experts from King's College London and regional clinical research hub to attend three MR Safety seminars.
 
Description RF safety management, measured by specific absorption rate (SAR) is particularly important for ultra-high field magnetic resonance imaging (MRI). An improved SAR calculation approach has been developed within this project.
The precise estimation of local SAR hot spots relies on the accurate knowledge of local tissue electrical properties, conductivity and permittivity, at the operating frequency. Therefore, real-time and subject-specific estimation of the electrical properties is crucial to the development of ultra high field MRI. Recent methods rely on information from the transmit magnetic field B1+ map in MRI to reconstruct the electrical parameters of the tissue. This makes use of an algorithm based on the finite-difference method where good accuracy of reconstruction can be achieved but this requires high-resolution B1+ maps to be used in order to reduce numerical artifacts, especially at the boundary between different media.
In this project, we have developed for the first time an algorithm which uses an interpolation scheme for poorly resolved two-dimensional B1+ map of the MRI scan to obtain fastly the high-resolution field maps for accurate SAR calculation via finite difference method.
With the resulting approximated high-resolution B1+ map, the accuracy of the electrical property calculation can be improved, especially over the geometry boundaries. The method was applied to reconstruct the conductivity and permittivity of a double layer cylindrical phantom model of known properties using low resolution B1+ map generated from three-dimensional electromagnetic simulation of a single channel bird-cage RF coil to mimic operation at 3 and 7 T imaging. Results were compared to those obtained from conventional finite difference method-based reconstruction from high resolution field maps, proving that the proposed algorithm is a valid tool.
The gain is a significant reduction of valuable MRI scan preparation time which allows a fast and reliable real time subject-specific SAR calculation.
Exploitation Route The algorithm is intended to be made public so that it could be used by clinical and academic researchers in the field of ultra-high MRI.
The source code will be made accessible as soon as the journal paper reporting the method and validation results on the double layer phantom model (title:"Fast estimate of the SAR using spline interpolation approach for B1+ mapping in Ultra-High Field MRI" is published.
Sectors Healthcare

 
Title Dataset for Fast Estimate of the SAR Using Spline Interpolation Approach for Parallel Transmit B1 Mapping in Ultra-High Field MRI 
Description This data is generated using a novel numerical technique based on spline interpolation developed for the project "Towards Patient Specific and Reliable SAR Management for Parallel Transmit Technology on Ultra High Field MRI". The aim of research and detailed research methodology are discussed in the corresponding paper "Fast estimate of the SAR using spline interpolation approach for parallel transmit B1 mapping in ultra-high field MRI". The detailed information about the dataset can be found in the figure caption respectively in the corresponding paper. This research work was carried out from June 2014 to August 2015. Access to the dataset can be allowed on request. 
Type Of Material Database/Collection of data 
Provided To Others? No  
Impact The dataset has been used to produce the figures for the journal publication "Fast estimate of the SAR using spline interpolation approach for parallel transmit B1 mapping in ultra-high field MRI". The journal paper is under preparation for publication in IEEE Trans. on Medical Imaging. There has been no notable impact yet from the publication of the dataset. It is anticipated that impact will follow the publication of the corresponding journal paper.